Loop antenna system



March 29, 1949.

L. L. LIBBY LOOP ANTENNA SYSTEM Filed March 12, 1945 2 Sheets-Sheet 1.wmllg tIIIIIIIIIII'IZIIIIIIII:I

INVENTOR. 1.55727? 1... L/BB) ATTORNEY March 29, 1949.

Filed March 12; 1945 L. L. LIBBY i LOOP ANTENNA SYSTEM 2 Sheets-Sheet 2IN V EN TOR. LESTER A. 4/55) A 77' OIP/VEY Patented Mar. 29, 1949 LOOPANTENNA SYSTEM Lester L. Libby, East Orange, N. J., assignor to FederalTelephone and Radio Corporation, New York, N. Y., a corporation ofDelaware Application March 12, 1945, Serial No. 582,207

i 19 Claims.

The presentinventi-on.relates to loop antenna systems including the loopantennas, their transmission line and terminations, and particularly toshielded loop antenna systems which are adapted to operate'over a broadband of the higher frequencies.

In shielded loop antenna systems, resonances occur at variousfrequencies which materially affect the response and phase of thesystem. These resonances interfere with normal operation and areespecially undesirable in certain systms, such as direction findingsystems.

In the absence ofthese'resonance effects, the theoretical response curveof such systems is considerably lower at the lower end of the band offrequencies than at the higher frequencies. The shape of this responsecurve isprimarily caused by the change in eifective height of the loopantennas over the band of frequencies involved. The response at thelower end of the frequency band is usually unsatisfactory. Furthermore,in view of the shape of the curve, it becomes necessary, in order toprovide for uniform operation over the entire frequency band, to providesome compensation such as for example, in the transmitter or receiverassociated with said antenna system.

An object of the present invention is the provision of animprovedantenna system, particularly of the shielded loop type.

Another object of the present invention is the provision of a shieldedloop antenna system in which the response is approximately flat over awide band of frequencies.

Another object of the present invention is the provision of a shieldedloop antenna system in which the eifects of resonances are substantiallyminimized.

Still another object of the present invention is the provision of ashielded loop antenna system whose response at the lower frequency isincreased whereby the entire .response curve over a broad band becomesflatter.

A further object of the present invention is the provision of animproved direction finding antenna system including shielded loopantennas and a sensing antenna.

Other and further objects of the present invention will become apparentand the foregoing will be best understod from the following descriptionof embodiments thereof, reference being had to the drawings, in which:

Fig, l is a grammatical illustration of a collector system for adirection finder incorporating an embodiment of my invention;

Fig. 2 isa view of the portion of the system illustrated in Fig.1,illustrating a modification thereof;

Fig.3 is a set of curves used in describing my invention; and

Figs. 4-7are equivalent circuits used in describing my invention.

Referring now to Fig. 1 the collector system there illustrated includestwo shielded p antennas l and 2 which are preferably arranged at rightangles to each other. Each of said loops is preferably in the'form of acoaxial line having an inner loop antenna 3 and an outer shield 4 withagap 5 at the top thereof, or otherwise symmetrically disposed inrelation to the two halves of the shielded loop antennas Shielded loopantennas I and 2 are coupled by means of dual transmission lines 6 and Irespectively to a goniometer arrangement 8. Said dual transmission lines6 and 1 are terminated in resistances 9 and I9 respectively andprimaries H and I2 respectively of a goniometer !3, the rotating coil I4 of which is coupled to the rotating primary I5 of a transformer l6whose stationary secondary H is coupled by means of a transmission lineH! to a direction finding receiver IS. The rotating primary l5 and therotating coil M are driven by motor 58.

In Fig. 1 there is also illustrated a sensing arrangement '20 comprisinga monopole 2! arranged above a counterpoise 22, the monopole andcounterpoise being connected respectively to opposite ends of theprimary 23 of a transformer 24 which may be provided with anelectrostatic shield 25 arranged between its primary 23 and itssecondary 26. The secondary 26 serves as a dummy loop and has the sameconstants as such a loop as nearly as they can be approximated. Thesecondary 26 is in turn coupled through a circuit 21 hereinafter to bedescribed, to a balanced dual transmission line 28 terminating in thedirection finding receiver 19 in a terminating resistance 29 and theprimary 39 of a transformer 3|, the secondary 32 of which is connectedin the usual manner in the direction finding receiver.

The physical arrangements of the two shielded loops 1 and 2 and thesensing monopole 2i and counterpoise 22 may be such as is described inthe copending application of H, G. Busignies-P. J. VanderWoude, for Loopantennas, Serial No. 478,083, filed March 5, 1943, Patent No. 2,419,673issued April 29, 1947, and T. H. Clark-P. J. VanderWoude for Loopantenna construction, Serial No. 478,082, filed March 5, 1943, PatentNo. 2,419,539 issued April 29, 1947.

In operating systems of the type hereinabove described over a broad bandof frequencies, resonances appear at various points and the response atthe lower frequencies is relatively poor. The response curve of thesesystems is therefore undesirably irregular. In accordance with myinvention, I provide reactive and resistive means for producing arelatively flat response curve over the entire band of frequencies atwhich said systems are designed to be operated.

In order to attenuate the effect of resonances in this system, I provideat the gap of each loop a resistor 33 connecting together the ends ofthe shield at said gap. While the value of the resistor 33 will varydepending upon the particular parameters of the system involved and thefrequencies at which it is operated, the value of such resistor ispreferably of the order of four to ten times the surge impedance of theline forming the shielded loop antenna.

In order to compensate for the poor low frequency response of thesystem, I provide in each of said shielded loop antennas a condenser 34positioned at the gap and in series with the inner conductor of loopantenna 3. The capacitor 34 preferably has such a value that at thehigher frequencies it merely serves as a by-pass condenser having littleif any substantial reactance, while at the lower frequencies, saidcapacitor has a substantial reactance and serves to tune the systemtoward the lower end of the frequency band.

While in Fig. 1, I have shown a single capacitor disposed at the gap andtherefore symmetrically arranged in regard to both halves of theshielded loop antenna, it is also possible to utilize two.

condensers 35 and 36 (see Fig. 2) in place thereof and'symmetricallydispose these two condensers in the entire shielded loop antenna system,that is, the two condensers may be arranged at symmetrical points withinthe shielded loop itself or L may be arranged as illustrated in Fig. 2at'the point of connection between the transmission line 6 and theshielded loop or may be arranged inside the transmission line. Whereversaid condensers 35 and'36 are placed, they must be placed in a similarposition, or symmetrically arranged, otherwise the balance of the systemwould be upset. Condensers 35 and 36 will each have a value equal totwice the value of condenser 34 if they are arranged close to the gap,but this value will vary the farther they are from said p.

In order that the sensing system should suitably track with the shieldedloop antenna system, the circuit 21 is made similar electrically to theshielded loop antenna arrangement by providing therein a shunt resistor3'! equivalent to resistor 33 and series capacitors 21 and 39 similar tocondensers 35 and 36 of Fig. 2 or equivalent to condenser 34 of Fig, 1.Condensers 27 and 39 will have the same values as condensers -35 and 36and twice the value of condenser 34.

Referring now to Fig. 3 in which relative response is linearly plottedalong the Y axis while the frequency in megacycles is logarithmicallyplotted along the X axis, curve 40 is a theoretical curve indicatingroughly what the reponse of a system, similar to the one heretoforedescribed, would be if my invention were not incorporated therein andfurthermore if there were no reso nances in the system. Curve 4|indicates what might be the response in such system due to theresonances. Curve 42 indicates the effects on the peaks and troughs ofcurve 4| produced by using damping resistors in the loop antenna systemis improved, it is still quite irregular. resistor 33 as arranged inaccordance with my invention, a response curve 49 may be obtained.

" It will be noted that this curve is substantially 'flat at the higherfrequencies but dips downward at the lower frequencies indicated by thenumeral 5|]. By using a capacitor such as condenser 34 of Fig. 1, orcondensers 35 and 36 of Fig. 2, a curve such as 5| will be obtained. Itwill be noted that curve 5| is substantially flat over the entirefrequency band covered. In the absence 'of'shunt resistor 33, 9. peaksuch as 52 would-be introduced by condenser 34'.

The manner inwhich the results described hereinabovezin connection withthe curves of Fig. 3 are produced, will be more readily understood froma study of the equivalent circuits of Figs. 4-7 in which the inductance53 is the equivalent of the inductance of the shield of the loops, theinherent capacity 54 is the capacity between the ends of the shieldacross the gap, and the impedances ZA and Zn are'the impedances of thissystem looking inwardly into the gap' in the directions indicated by thearrows A and B in Fig. l.

1 Referring now to Fig.4, the'resistance 55 is the equivalent resistanceof the series resistor described in my co-pendingapplication, L. Libby-2, for Antenna systems, Serial No. 582,206,'filed March 12, 1945, PatentNo. 2,419,577, issued April 29,1947. It will be seen that in thearrangement of Fig. 4, the voltage-drop across ZA, ZB and resistor'55will'vary with frequency and since Zn and Z8 will become relatively-veryhigh in value in comparison with resistor 55 at certainresO-nance-points, these will primarily determine the response andwill'therefore vary this response according to the frequencies involved.

'Referring'now to Fig. 5, it will be seen that resistor 33, as arrangedin accordance with my invention, is in shunt with impedances ZA and Z3and when said impedances become greater than resistor 33, resistor 33will primarily control the voltage built up across inductance 53 andtherefore determine the response-of the system. Consequently resistor 33will greatly damp the peaks and troughs due to the resonances. V

In Fig. 6 it will be seen that the condenser 34 appears between ZA andZ3. This condenser is the equivalent of the condenser 34, Fig. 1, anddue to its value will cause the circuit to resonate towards the lowerend' of the frequency band and consequently will produce-'a greaterresponse at that end of the band. I V @Fig. 7' isthee'quivalent "circuit'of' the modification of Fig. 2. Because ofth'e positionof thecondensers 35-and 36, the 'si'des of the shielded loop will appear ascoaxial transmission lines'53 and 51 separating condensers 35 and 36from the inductance 53 of the shield and the impedances then will be theimpedances Z0 and Zn of each half of the dual transmission line 6looking into said transmission line towards thetermination thereof. Itwill be seen that thecircuit of Fig. 7 will function in the same manneras the-circuit of Fig. 6 but that the exact position of thec'ondensersand 86 will change the various values and in actual construction itbecomes necessary to change the value of these condensers, and.sometimes other parameters of the system, in accordance with theposition of said condensers. While I have described 'my invention indetail in connection with the embodiments of Figs. 1 and 2, it will beapparent that numerous changes inay be made in the details of myinvention as well as the system in which it is employed. For example, myinvention may be used with other than direction finding systems. Theshielded loops could be made rotatable and the oniometer dispensed with.Furthermore, while I have shown the loops as shielded loops, it will 'beapparent lower frequencies of operating band. Furthermore while I haveshown the sensing arrangement as a balanced arrangement (having mirrorsymmetry) includin a dual transmission line, a balanced termination, apair of symmetrically arranged condensers, and a balanced secondaryacting as a dummy loop, it will be apparent to those versed in the artthat an unbalanced arrangement, using a coaxial line, a single-endedtermination, etc., and acting as a half of the symmetrical arrangement,might be employed in place thereof, the operation thereof depending onthe mirror symmetry equivalence of balanced and unbalanced circuits.Other variations will readily occur to those versed in the art from theeachings of my invention.

Accordingly while I have described above the principles of my inventionin connection with specific apparatus, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and 1 not as a limitation on the scope of myinvention as set forth in the objects of my invention and theaccompanying claims.

I claim:

1. In an antenna system including a shielded loop antenna comprising aconductive loop and a shield surrounding said loop and having a gaptherein, a transmission line coupled at one end to said shielded loopantenna, and terminating means coupled to the other end of saidtransmission line; means for producing a relatively fiat response over abroad band of frequencies comprising reactive means and resistive meansmounted in series and in shunt respectively with said transmission line.

2. The antenna system accordin to claim 1 wherein said reactive means iscapacitive.

3. The antenna system according to claim 1 wherein said reactive meansis capacitive and is arranged in series in the loop.

4. The antenna system according to claim 1 wherein said reactive meansis capacitive and is in series with the loop and is arrangedsymmetrically in relation to both sides of the loop.

5. The antenna system according to claim 1 wherein said reactive meansis a condenser in series with the loop and positioned at said gap.

6. The antenna system according to claim 1 wherein said reactive meansis a condenser arranged in series in the loop and of such a value as tohave relatively large reactance at the lower frequencies of the band offrequencies at which said system is operated compared to its reactanceat the upper frequencies of said band.

7. The antenna system according to claim 1 wherein said reactive meansis a condenser arranged in series in the loop and of such a value as tohave negligible reactance at the higher frequencies of the band offrequenciesat which said system is designed to operate and to have asubstantial reactance at the lower frequencies of said band.

' 8. ,The antenna system according to claim 1 wherein said reactivemeans comprises a pair of condensers each arranged in series with theloop and positioned symmetrically with respect to the gap 9. The antennasystem according to claim 1 wherein said resistive means consists of aresistor positioned atthe gap and connecting the ends of the shield atsaid gap.

0. The antenna system according to claim 1 wherein said resistive meansis comprised of a resistor positioned at the gap and connecting the endsof the shield at said gap, the value of such resistor being of the orderof four to ten times the surge impedance of the line formed by the loopand its shield.

11. In an antenna system, a shielded loop antenna comprising aconductive loop and a shield surrounding said loop and having a gaptherein, a transmission line coupled at one end to said shielded loopantenna and terminating means coupled to the other end of saidtransmission line; means for attenuating the eiTects of resonances insaid system comprising a resistor positioned at the gap and connectingthe ends of the shield at said gap.

12. The antenna system according to claim 11 wherein said resistor has avalue of the order of four to ten times the surge impedance of the line-formed by the loop and its shield.

13. In an antenna system including a loop antenna in the form of acoaxial line comprising a conductive loop and a shield surrounding saidloop and having a gap therein, a transmission line coupled at one end tosaid loop antenna, and terminating means coupled to the other end ofsaid transmission line; means for increasing the response of said systemat one end of the band of frequencies over which the system is designedto operate comprising reactive means arranged in series with saidtransmission line.

14. The antenna system according to claim 13 wherein said reactive meansis capacitive.

15. The antenna system according to claim 13 wherein said reactive meansis capacitive and is symmetrically disposed in said system.

16. In an antenna system including a shielded loop antenna comprising aconductive loop and a shield surrounding said loop and having a gaptherein, a transmission line coupled at one end to said shielded loopantenna and terminating means coupled to the other end of saidtransmission line; means for increasing the response of said system atthe lower end of the band of frequencies over which the system isdesigned to operate comprising a condenser positioned at the gap and inseries with the loop.

17. In an antenna system including a shielded loop antenna comprising aconductive loop and a shield surrounding said loop and having a gaptherein, a transmission line coupled at one end to said shielded loopantenna, and terminating means coupled to the other end of saidtransmission line; means for increasing the response of said system atthe lower end of the band of frequencies over which the system isdesigned to operate comprising a pair of condensers each arranged inseries with the loop and positioned symmetrically with respect to thegap.

18. In a direction finding system comprising a pair of shielded loopantennas arranged at an angle to each other and each including aconductive loop, a shield surrounding said loop and having a gaptherein, a pair of transmission lines each coupled at one end to one ofsaid shielded loop antennas, a pair of resistive terminating means eachcoupled to the other end of each of said transmission lines, agoniometer having a pair of stationary coils each coupled to one of saidother ends of said transmission lines and a rotary coil, a directionfinding receiver, and means coupling said rotary coil to said directionfinding receiver; means for producing a relatively fiat response over abroad band of frequencies comprising reactive means and resistive meanscoupled in series with the loop and connecting the ends of the shield atsaid gap respectively.

19. In a direction finding system comprising a pair of shielded loopantennas each including a conductive loop, a shield surrounding saidloop and having a gap therein, a pair of transmission lines each coupledat one end to one of said shielded loop antennas, and a pair ofterminating means each coupled to the other end of each of saidtransmission lines, a sensing antenna arrangement including a dummyloop, a transmission line coupled at one end to said sensing antenna andterminating means coupled to-the other end of said last mentionedtransmission line, a direction finding receiver, and means coupling theother end of each of said transmission lines to said receiver; means forproducing a relatively fiat response over a broad band of frequenciescomprising separate reactive means and separate resistive meanspositioned'at each of said gaps and means for coupling each reactive andresistive means in series and in shunt respectively with itscorresponding transmission line, and additional resistive and reactivemeans coupled in shunt and series respectively with said sensingantenna. transmission line.

LESTER L. LIBBY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,736,268 Loughlin Nov. 19, 19292,326,945 Holsten Aug. 17, 1943

